Blasthole drills are large machines used in mining operations to drill holes for explosives. A conventional blasthole drill comprises a frame supported by crawlers for movement over the ground, and a mast supported by the frame for movement between a substantially vertical position and a number of angled or non-vertical positions. The mast defines a drill hole axis. A rotary head moves relative to the mast along the drill hole axis. The rotary head engages the upper end of a drill pipe for rotating the drill pipe and driving the drill pipe into the ground. When drilling a blasthole that is deeper than the height of the mast, more than one section of drill pipe must be used. After the first section of drill pipe is driven into the ground, the rotary head moves back to the top of the mast and another section of drill pipe is connected to the top of the first section. The rotary pipe then drives the second section into the ground. It is not unusual to use four sections of drill pipe. Such a deep blasthole is referred to as a "multi-pass" blasthole. The various sections of drill pipe are supported relative to the mast and moved into position (on the drill hole axis) at the appropriate time by a device known as a pipe rack.
When drilling a multi-pass blasthole, a considerable amount of time is spent in adding and removing individual drill pipe sections from the drill string. Also, when drilling blastholes at an angle, a considerable amount of time is spent engaging and disengaging a device known as a pipe positioner, which aligns and supports the lower end of a suspended pipe section (i.e., maintains the lower end of the pipe section on the drill hole axis) during connection and removal from the lower pipe string. Thus, a considerable amount of time is spent doing things other than drilling.
A carousel-type drill-through pipe rack is known. The carousel rack includes a circular plate which rotates about its center and which supports the lower ends of a plurality of drill pipes. The carousel must be extended to locate a pipe on the drill hole axis and must be retracted to allow the rotary head to pass.